Process of carburizing



Nov. 10; I953 w. E. HARBAUGH PROCESS OF CARBURIZING Filed Aug. 30, 1952 BYE. fii igugp Willis Patented Nov. 10, 1953 PROCESS OF CARBURIZING Willis. E21. Harbaugh; Bareville,--, Pa assignorg 150:; Radio :Corporationaof America -1a corporation .of

Delaware; 1

Application'Augl lstfi), 1952; Serial No. 307,193

loyclaimsrv (Cl.

This" is acontinuation-in part of my cO-D ing-- application, Serial'No-i 165;093;"fi-led- May" 29; 1950; now abandoned; V

Mypresent invention relatesto the carburizingof'metallic surfaces and'more-particularly to 5a a process for. carburizing the surfaces -of thor1ated tungsten electrodes :of the type'used-in electron dischargedevices; I

Intthe manufacture" of electron discharge devices, particularly those designed fOIijhighIJQWtBIX output considerabledifficulty has been met-incidtaining v carburizedi'thoriated tungsten electrodes" having, uniformly satisfactory, characteristics; One. serious disadvantagein the manufacture'of: I such electrodes has. been the.;.poor andunstable, emission obtained; Other-disadvantagesof QI'O'C: esses in ,use. have to. do, with the wide variationin the characteristics. of both the electrodes and the; tubes in which they are incorporated; Since-such, devices often use extremely high .values. ofivoltage and. current. the .foregoing ,critically, affect, satis factory operation thereof."

It; has been found. th'atthe. tungsten. carbide. layer on .thoriated tungstenelectrodes may. vary. in. crystalline structureiroma.massivejphase cor: responding. to. WQtoa..second massivenhasecorresponding .to,W2,C and thenthrougma. laminated phase in:. which. the; carbon; concentration... by weight pencentvariesirom; 3:.16.to-approximate1y q 2.45. In general, there wasifound to.be an abrupiav line-between therWC phase 6,.12. perwcentrcarbflns and the massive W2C phase, 3.16 per cent carbon... Asiindicated. in .the, article .Carbidestructures-in carburized thoriated. tungstenfilamentsii C'mWs 3 Horsting, J ournalof. Applied. Bhysicsavol. 1.81;No. 1, 95, J anuary- 1947., the.- laminated; phase-has. been-.iound to.beithepreierreds-form. of. tungsten: carbide; for thoriatedtungstezr emitters.-. How ever; known processes. of carhurizinghave failed,- to produceuniform results. and? have: required costly I activation at high-temperatures aftenv the: emitterhas-been mounted man-electron discharges device.

A pr-incipal object of'myinventionr'isthe provi=- sion of aprocess.- for manufacturing,- carburizedselectrodesha-ving improvedand stableemiss-iom Anotherobjectis the provision-.ot-an improved." process forv pre-carburizing thor-iated1- tungsten-.- electrodes. g

A further cbject is. the. provisionioila process-for precarburizingzv thoria-ted tungstens electrodes-; which insures. the-formationof-the desiredecrysa tallinephase. 1.

Yet another object. istthe. provision of? such? a:

bide layer. having atlaminars struct t advantages" which are? gained aswill be pfoint" method for precarburizingwhicn:permits tl'ieiuses oil. preferred electrode-J shapes and? struetures'rewhich will 2 have=fu:ll -stable--:emissionat the opera; ating temperature without requiring; high: te peratu-reactivation-after itais' mounted-in amvaca uumtube; I Still. another:objectds the@provisien :of a jiroicess;c for pre-carburizing thoriated tungsten electrom' emitters and. forming a preferred tun steri car- In carrying 4 out one-embodiment myvnovel; process, -I have heated-: ,tho-riated:tunestemeleca trode: inan inert "atmosphereeto recrystall ize the. tungsten. Then whilermaintain-i-n'g th'e temperaei ture of the.-electrode lEhavaintroduced hydrccar n hon-vapor intothe' atmosnheres Thevhydroea 1 bon-vapor con-tent.:of=tthe atmospherei-waszn-i ai :2 tained while theetemperature of the-electrodezwasi; maintained for a desired-neriod-of"time I-h'aves 20- then; raised the temperature oi-itheelectrodeycuh off the-.supplyofxthe hydrocarbon .vapcr;iand= maintainedithe electrode-at the zelevatedztemperaei ture"in-the atmosphereiniwhich theehydrocarb'on; vapor. concentratiene was rapidly:rlce'in'g depleted;-

have:- found-the:-carburizedathcriated"tungsteifi emitters thus formejd tmhavw the. desired lam-i nated phase crystalline structure; sums elec''- trodes" give uniform-i stable: electromemissionfiat the -operating.temperatureiwithout requiring-2mm:

ther activation treatmentl The novelfeatureswwhich:Iibelieve tozkie chanatacter-istic-.=of; my invention are set forth-with para-'1 ticularity: in the appended" claims: The?inve'en-"r tion:itselfz-wflhbiistbe understood "by: reference'Ttb': the following-. description taken ineconnectibn with the;-accompanying .drawing inwhich':

Figurerl :is. a-semiediagrammatic view *0f an-ap paratusfor carrying outamyiinvefntiong and Figure 2: is a e reproduction: of a phot'cirfiicro graph-showing a segment iofza-"thoriated -tungsten electrode magnifiedAOOJtimeS:

I-prefera'bly.'carryvoutamyFprocess and carburize the? electrodes: prior i'to r assembling: the sarn'e the-1'; mount or? supporting structure because: 0

out: However; my pro'c'e'ss' is? not limited to p'r'e carburizing and also lends itself to '-tl'ie "c-arb'uriz'-' ing of electrodes after' they are incorpg'ir ated-- 'n" the mount-istructure'of an electron dischargede f vice*and provides greatly 'improvedelectrodes:-

In Fig-ure' 1 I haveill'u'sti'ated 'oneform of *appa iratus: which: I have" conveniently used" tc--' pris carburizei a; thori'ated tungsten eieetmn emitter-- cnelectrode :I 0 h'aving reduced mortiohs- I =1 whiolf' function. asvheatj losse: correction nieans- -and mounting heads 12. Electrode i is conveniently mounted by means of its mounting heads 12 between two slotted and grooved supports [3, 14 which are each supported from electrically conductive members l5, IS in any suitable manner. Members i and it; are connected to the terminals of a source of electrical power (not shown) by leads ll, 18. Member i5 is insulated from standard l9 and supported thereon. Member 16 is conveniently slidably connected to standard I9 in such manner as to permit axial elongation of electrode II) when heated.

Cooling means in the form of tubes 20 connected to a controlled source of dry hydrogen direct the coolant which is inert with respect to thoriated tungsten, at reduced portions H and serve to maintain portions l l at a sufliciently low temperature to prevent their burning out or being carburized.

Bell 2| is movably mounted and may be lowered around standard 19' and the parts supported thereon with its lower end 22 open to the atmosphere if a light gas is used; with a heavy gas, filling from the bottom would be necessary. Tubing 29 opens into bell 2! and is connected to controlled sources of gases to be introduced into the bell to form an atmosphere most favorable to carburizing.

As pointed out above, it has been found that the laminated phase of the tungsten carbide structure is that desired while the massive WC and W2C represent undesirable constituents and are to be avoided if the desired emitter is to be obtained. In Figure 2 is shown a segment of a carburized thoriated tungsten electrode as reproduced from a photomicrograph and is a magnification of 400 times. All of the lines 23-26 represent boundaries of the photomicrograph while surface 2'! is a reproduction of a portion of the outer surface of the electrode. Line 28 running through the specimen shows the sharp interface between the core and the carbide in the finished product. Core 3| has as shown the recrystallized structure of thoriated tungsten while the laminated phase of tungsten carbide is clearly apparent in the region 30 between interface 23 and exterior surface 21.

In accordance with my improved process I consistently form the preferred laminated phase of tungsten carbide ranging from 3.16 to 2.45 per i cent carbon. To attain these results I have found it to be essential that the temperatures, hydrocarbon vapor concentration and duration of heating be critically controlled. If the temperature to which the electrodes is heated is too low, a shell of carbon forms on the thoriated tungsten electrode which gives it undesirable characteristics. If the temperature is too high, the depth of penetration of the carbide may be excessive while a surface layer of decarburized thoriated tungsten may be formed depending upon the carbon content. Too great a concentration of hydrocarbon vapor prevents the formation of the preferred laminated phase and results in either the massive W2C or WC phases. Too small a concentration generally results in a decarburized or carbon-starved surface layer with an internal layer of the carbide which does not reach the surface. The length of the heating time interval is also critical. If heating is too prolonged carbide penetration will be excessive. Since tungsten carbide is extremely brittle this condition must be avoided. On the other hand, if the heating period is too short the depth to which the electrode is carburized will be insuflicient to pro- '4 duce the desired effect. The carburizing reaction is extremely rapid at elevated temperatures and must be carefully regulated.

I have found that an initial heat treatment of approximately 20 to 40 seconds at a temperature between 2575 degrees and 2625 degrees K. serves to recrystallize and stabilize the tungsten sufficiently to aid in obtaining a much more uniform product. This is carried out in an inert atmosphere. By inert or inert atmosphere, I wish it to be understood here, as elsewhere, that the gas or vapor does not react unfavorably with thoriated tungsten and is substantially inert with respect thereto. Though the heat treatment is highly beneficial, it is not essential and may be omitted.

On completion of the recrystallization and without allowing the electrode to cool hydrocarbon vapor is admitted by introducing a gas mixture of hydrocarbon with an inert gas. Heating of the electrode is continued at a temperature between 2575 degrees and 2625 degrees K. for one minute. During this interval the hydrocarbon is broken down to hydrogen and carbon; the carbon combining with the hot tungsten to form a surface layer of tungsten carbide. This carbide is of the massive W2C type.

I then raise the temperature of the electrode substantially to between 2775 degrees and 2825 degrees K. and simultaneously cut off the supply of hydrocarbon vapor but continue to admit the inert gas. The electrode is maintained at this temperature for about two minutes in the rapidly waning hydrocarbon atmosphere. This insures that this part of the reaction is one which is carbon-limited. The penetration of the carbide layer becomes greater. The percentage composition of the carbide layer becomes reduced below 3.16 per cent carbon and to the range of between 2.45 and 3.16 per cent carbon; thus forming the laminated phase.

It is to be understood that substantial variations in the cross-sectional thickness of the metal being carburized may require or permit corresponding changes in the duration of the heating times; temperature being held constant. However, the variations may be readily determined.

Hydrocarbons other than benzene which may be mixed with hydrogen to form a carbon bearing vapor include acetylene, toluene, and xylene. Non-hydrocarbons which may be mixed with hydrogen to form a carbon bearing vapor include colloidal particles of carbon, aniline, and diethyl ether.

Other hydrocarbon and non-hydrocarbons may be mixed with the hydrogen to form a carbon bearing vapor provided they vaporize readily, decompose into their elements to produce free carbon at the carburizing temperature of the electrode, and do not have other elements, which adversely affect the electrodes being carburized.

As a specific example, thoriated tungsten electrodes I U have been mounted in the apparatus of Figure 1 and pre-carburized. Electrodes It were substantially trapezoidal in cross section having a width of .057 inch narrowing to .040 inch with the narrow face rounded. The thickness from front to rear was .037 inch. The electrode was approximately 8.6 inches long. The reduced portions H are cooled :by jets of pure dry hydrogen as shown in Figure 1. Bell 2 I, 24 inches high and 14 inches in diameter, was lowered and the electrode heated for thirty seconds at 2600 degrees K. in an atmosphere of pure dry hydrogen which was continuously flushed through the bell at the rate of 40 cubic feet per hour; the end assegsee 22 of" the I pen ="being 'ofien' the-atmos o f' l'iydrogen the 40%ub'ic feet per: was flowing thr'oug maintar 01 ute.

ofber'izenewaiiorirra'car had an intemardiameter f pool of benzene at room temperaturei "The hy (inseam-retrain.surreasre bom .200 in. internal diameter tubing with their openings 5 inches Cathodes having thoriatedtungsten emitters? carburiz ed.="-by my process haveuniformz-stable electron emission at the operating temperature of 2000 degrees K. and do not require further activation. Though I have described my novel process in connection with one specific type of cathode element which is particularly useful in super power devices, cathodes have also been successfully made in accordance with my process having entirely different shapes or structures. In such case, the duration of the heating is modified in accordance with the cross-sectional thickness of the cathode or elements and the extent of carbide penetration desired. Though gases or vapors other than hydrogen or benzene may be used, I prefer these as most consistently providing a uniform stable electrode. It is apparent, therefore, that many variations may be made without departing from the scope of my invention as set forth in the appended claims.

I claim:

1. The process of carburizing a thoriated tun sten element, comprising heating the thoriated tungsten element in a carburizing atmosphere containing inert gas and hydrocarbon vapor at sufficiently high temperature to decompose the hydrocarbon, and then continuing the heating of the thoriated tungsten element at a higher temperature in an atmosphere in which the hydrocarbon content is rapidly decreasing.

2. The process of carburizing a thoriated tungsten element, comprising heating the thoriated tungsten element to a predetermined temperature in an atmosphere containing inert gas to which hydrocarbon vapor is being continuously added, and then stopping the addition of the hydrocarbon vapor and continuing the heating of the thoriated tungsten element at a higher temperature in said atmosphere in which the hydrocarbon content is rapidly decreased.

3. The process of carburizing a thoriated tungsten element, comprising heating the thoriated tungsten element in an inert atmosphere at a predetermined temperature and stabilizing the crystalline structure, then while continuing to heat said element at said temperature adding hydrocarbon vapor to said atmosphere said temperature being sufliciently high to decompose the hydrocarbon to create and maintain a carburizing atmosphere, and finally discontinuing the adphere:

hydrogerr bypassing. the 1 '12- cubic fee't -"per hour: of :"h'ydrogen first: through-a d'ry erand therfthrbugh anatmo'sphere 'retorrThe carhuretor A; inche's witli a ditiomofi hydrocarborrivaponiand heat 'saidel'ement'iattazsubstantiallyfhigherte perature whiliarthe' ydrocarbon: vapori-cont'ezit" of ith'e atmosphere?eisirapidlyibeingzdepleted. 1

' 4: Theimethodaofccarburizing athbriatedstung'e sten-: elementy-fcomprising heatingz'saidthoriated; tungsten i element. at a::pred'eterminedrtempera' ture'fin carburizing atmosphere? toiwwhichicare bon bearingi vaporfisadded itdmaintainiithe. car; bon: concentration; and'iforming iaz.:sl'ayer :"on: said el'ement' wliich i'ssubstantially 'the massivezphaser; saicf temp'eraturei being highienoughztodes: compose saidrvaporfandxtnen reducing fthelfaiddi r tionrof'fcarbom bearingztvaporz'and continuingtoz heat said'. elementziinzsaidi atmospheretim which; the carbomrconten srammy; reduced; and cone ly allofl'saidwzQ to theslamie- 'I'li'ein'ieth0d:of: prei-carburizingt; azthoriatedi tungsten-element, comprisingzheating saiidzthori:. at'e'di tungsteii 'elementi at ai'predeterminedsteme perature irr-Jan atmosphere containing-:inert'gas? tdwhich carbonibearingivapor:isbe'ingaddd and; forming a surface layer of a predetermined thick;

ness which"issubstantially entirelyimassivezphase tungsten carbide'i'on saidielemenm.said-temperate ture:- bingrhigh ."enouglrito: .decomposezsaidrvapors and-themdecreasing?thesaddition of. carbombear;-= ing vapor and continuing to heat said element in said atmosphere the carbon content of which is rapidly decreasing and converting substantially all of said massive phase tungsten carbide to a layer Of greater thickness of substantially all tungsten carbide of laminated phase.

6. The method of pre-carburizing a thoriated tungsten element having a portion of reduced thickness, comprising heating said thoriated tungsten element at a predetermined temperature in an atmosphere containing inert gas to which carbon bearing vapor is being added and forming a surface layer on said element which is substantially all massive phase tungsten carbide while preventing the formation of such a layer on said portion of reduced thickness, said temperature being high enough to decompose said vapor and then reducing the addition of carbon bearing vapor and continuing to heat said element at a higher temperature in said atmosphere in which the carbon content is rapidly reduced and converting substantially all of said tungsten carbide to the laminated phase, while at the same time continuing to prevent the formation of a carbide layer on said reduced portion.

7. The method of carburizing a thoriated tungsten element, comprising heating said thoriated tungsten element to a temperature between 2575 degrees K. and 2625 degrees K. in an atmosphere containing an inert gas and to which a hydrocarbon vapor is being added and forming a sur face layer on said element of predetermined thickness which is substantially entirely massive phase W2C on said element, and then stopping the addition of hydrocarbon vapor and heating said element to a temperature between 2775 degrees K. and 2825 degrees K. in said atmosphere, the hydrocarbon content of which is rapidly decreasing and converting substantially all of said layer of W2C to a layer of greater thickness of laminated phase W20.

8. The method of carburizing a thoriated tungsten element comprising heating said thoriated tungsten element to a temperature between 2575 degrees K. and 2625 degrees K. in an atmosphere containing dry hydrogen and to which benzene vapor is being added and forming on said element a surface layer of predetermined thickness which is substantially entirely massive phase W2C, and then stopping the addition of benzene vapor and heating said element to a temperature between 2775 degrees K. and 2825 degrees K. in said atmosphere the benzene vapor content of which is rapidly decreasing and converting, substantially all of said layer of W2C to a layer of greater thickness of laminated phase W20.

9. The method of carburizing a thoriated tungsten element comprising heating said thoriated tungsten element to a temperature of about 2600 degrees K. in an atmosphere containing dry hydrogen and to which benzene vapor is being added and forming on said element a surface layer of predetermined thickness which is substantially entirely massive phase W2C, and then stopping the addition of benzene vapor and heating said element to a temperature about 2800 degrees K. in said atmosphere the benzene vapor content of which is rapidly decreasing and converting substantially all of said layer of W2C to a layer of greater thickness of laminated phase W26.

10. The method of pre-carburizing a thoriated tungsten element having a portion of reduced thickness, comprising heating said thoriated tungsten element at a predetermined temperature in an atmosphere containing inert gas to which carbon bearing vapor is being added and forming a surface layer on said element which is substantially all massive phase tungsten carbide while cooling said portion of reduced thickness and preventing the formation of such a layer on said portion of reduced thickness, and then reducing the addition of carbon bearing vapor and continuing to heat said element at a higher temperature in said atmosphere in which the carbon content is rapidly reduced and converting substantially all of said tungsten carbide to the laminated phase, while at the same time, continuing to prevent the formation of a carbide layer on said reduced portion.

WILLIS E. HARBAUGH.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,663,457 Gero Mar. 27, 1928 1,695,819 ONeill Dec. 18, 1928 2,400,893 Thurber et a1. May 28, 1948 FOREIGN PATENTS Number Country Date 310,103 Great Britain Oct. 2, 1930 

1. THE PROCESS OF CARBURIZING A THORIATED TUNGSTEN ELEMENT, COMPRISING HEATING THE THORIATED TUNGSTEN ELEMENT IN A CARBURIZING ATMOSPHERE CONTAINING INERT GAS AND HYDROCARBON VAPOR AT SUFFICIENTLY HIGH TEMPERATURE TO DECOMPOSE THE HYDROCARBON, AND THEM CONTINUING THE HEATING OF THE THORIATED TUNGSTEN ELEMENT AT A HIGHER TEMPERATURE IN AN ATMOSPHERE IN WHICH THE HYDROCARBON CONTENT IS RAPIDLY DECREASING. 